JOHANNESBURG: Nelson Mandela's close family members gathered to hear a sombre prayer wishing the anti-apartheid icon a "peaceful, perfect, end" as he lay in hospital in a critical condition with life seemingly slipping away.

Cape Town Archbishop Thabo Makgoba visited Pretoria's Mediclinic Heart Hospital on Tuesday to pray with wife Graca Machel "at this hard time of watching and waiting".

Mandela, a Nobel Peace Prize winner, who spent 27 years behind bars for his struggle under white minority rule and went on to become South Africa's first black president, was admitted on June 8 with a recurrent lung infection.

During a 19-day vigil his family, and the world, have watched as the 94-year-old slipped from a stable to a critical condition.

Many now fear for the man who, in defeating apartheid, bent the arc of history to his will and lit a fire that shone for millions across the world.

The archbishop's prayer seemed to echo a growing feeling of inevitability about Mandela's condition that is increasingly voiced by South Africans, to whom he remains a moral giant, even though he stepped back from public life a decade ago.

"May (we) be filled with gratitude for all the good that he has done for us and for our nation, and may (we) honour his legacy through our lives," the prayer read.

"Grant Madiba eternal healing and relief from pain and suffering," it said, referring to Mandela by his clan name.

"Grant him, we pray, a quiet night and a peaceful, perfect, end."

The police presence in the neighbourhood of Mandela's home was bigger early Wednesday, compared to previous nights, South Africa's SAPA news agency reported.

Outside the hospital in Pretoria flowers and messages of support piled up as a growing crowd sang hymns.

"I'm here because of Tata (father) Mandela," said 38-year-old Tolly Mogane, one of generation who Mandela led from the darkness of racist rule to democracy.

"Today I'm free, nobody is going to tell me to go home."

As South Africans have become resigned to Mandela's mortality, his role in shaping their nation has come into sharper focus.

"Mr. Mandela set the example," said Rosemarie van Staden, who was handing out muffins to well wishers. "He wanted us to be a rainbow nation and we really must carry on."

Meanwhile members of Mandela's extended family huddled at his rural homestead to discuss the days to come.

One of Mandela's daughters and at least two grandchildren were seen gathering for a meeting in the village of Qunu, where the charismatic former leader spent his childhood tending cattle and living in mud-walled huts.

The meeting was called "to discuss delicate matters", amid speculation that the location of his eventual grave was on the agenda.

Tensions emerged between elder clan members and Mandela's grandchildren during the talks, according to sources close to the family, although it was unclear exactly why.

President Jacob Zuma called on South Africans to respect the Mandela family's "dignity and privacy".

The White House said it was monitoring Mandela's condition but could not yet say whether his ill health would affect a planned visit by US President Barack Obama to South Africa from Friday as part of a tour of Africa.

In any case, Obama is unlikely to see Mandela as he is "indisposed", South Africa's international relations minister Maite Nkoana Mashabane said.

Mandela -- who is due to celebrate his 95th birthday on July 18 -- has been hospitalised four times since December.

Upon his release from jail in 1990 in one of the defining moments of the 20th century, Mandela negotiated an end to apartheid and won the country's first fully democratic elections.

He served a single term as president, guiding the country away from internecine racial and tribal violence, before taking up a new role as a roving elder statesman and leading AIDS campaigner.

He stepped back from public life in 2004.

The South African government has been criticised following revelations that the military ambulance that carried Mandela to hospital developed engine trouble, resulting in a 40-minute delay until a replacement vehicle arrived.

The presidency said Mandela suffered no harm during the wait for another ambulance to take him from his Johannesburg home to a specialist heart clinic in Pretoria 55 kilometres (30 miles) away.